Oracle collects Active Session History (ASH) statistics (mostly wait statistics for different events) for all active sessions every second from v$session and stores them in a circular FIFO buffer in the SGA. ASH records are very recent session history within the last 5-10 mins. The MMNL (Manageability Monitor light - shows up as "Manageability Monitor light 2" process) process, if the buffer fills up before the AWR flushes (by default every hour) the MMNL process will flush the data to disk (data stored in dba_hist_active_session_history).